How Our Brain Controls Movement
Damian Lillard of the Portland Trailblazers hits a game-winning step-back jumper to eliminate the Oklahoma City Thunder from the NBA playoffs (link). This is a remarkable feat considering the movement coordination required to perform the action.
How is it that Lillard was able to shoot the basketball into the hoop?
Figure 1: Damian Lillard’s game winning 3-pointer against the Oklahoma City Thunder
Source: https://www.theringer.com/nba/2019/4/24/18513871/winners-losers-portland-trail-blazers-oklahoma-city-thunder-game-5-damian-lillard
Likewise, how can we walk up and down the stairs, throw a ball, or go for a run? How are we able to control our movement?
In short, when we decide on an action or motion that we want to perform, our brain sends a signal to our muscles and tells them to pull our bones in a certain direction so that we can do that movement. To dive deeper, we’ll first need to examine the structure of our muscles, and then discuss how our brain tells our muscles what to do.
Muscle Physiology
We have three types of muscle: cardiac, smooth, and skeletal. Skeletal is the type of muscle we’re all familiar with and drives our movement throughout the world. It’s easiest to think of muscles as tubes inside tubes.
Figure 2: Structure of a muscle
Source: https://www.brainkart.com/article/Structure-of-a-skeletal-muscle(Voluntary-muscle)-fibre_33243/
Starting from the outside, we have the entire muscle itself. The whole muscle is made up of muscle fascicles, and the muscle fascicles are made up of 100s of muscle fibers, which are the individual muscle cells. The whole muscle is the biggest tube, filled with small tubes called fascicles, which are filled with even smaller tubes called fibers.
When your brain activates these muscle fibers (more on this later), they contract and shorten, causing your bones to move! If you look at Figure 2 below, the biceps will contract and pull your forearm towards your upper arm. Furthermore, muscles can ONLY shorten themselves… we can’t make them any longer than they normally are. So how are you able to straighten your arm out again after you flex your biceps? You are able to straighten the arm because you also have triceps on the backside of your arm! When these muscles contract, they pull your forearm back down into the straightened position.
Figure 3: The biceps muscle, which originates from the shoulder and attaches to the forearm.
Source: https://www.ck12.org/book/ck-12-biology-advanced-concepts/section/17.30/
Consider what would happen if both the triceps and biceps muscles contracted at the same time, with the same amount of force. Our forearm wouldn’t move at all, as the two muscles would be contracting against each other! Luckily, when we want to move our arms in space, our brain can simultaneously send signals to contract one set of muscles, while relaxing the opposing set.
Neural Signals
How exactly do our brains make our muscles contract? The cells in our brain, called neurons, are capable of generating electrical signals that get sent out to our muscles. The large spike in electrical activity is called an action potential [2], and neurons use this to communicate information. When you want to move your hand to grab a pencil, or move your leg and foot to take a step, your brain sends these electrical signals to your spinal cord. Your spinal cord then sends them to your muscles where chemicals are released, causing them to contract.
Figure 4: A single motor unit connected to multiple muscle fibers
Source: https://training4endurance.co.uk/neuromuscular-coordination/
Let’s return to the simple example of moving your forearm by contracting your biceps. Our bodies have specialized cells that make a connection between our spinal cord and our muscles, which are called motor neurons [3]. The electrical signals from our brain are sent to the body by motor neurons and reach the location where the neuron connects with individual muscle fibers. A single motor neuron can be connected to multiple muscle fibers, which is called a motor unit. When an action potential travels down a motor unit all the muscle fibers that receive the electrical signal briefly shorten, or contract [1]. When many action potentials are sent to the fibers, we begin to contract our entire bicep, causing us to bend our elbow!
We have millions of these motor neurons connected to hundreds of muscles, each with thousands of individual muscle fibers! Our brain has the amazing ability to efficiently coordinate our movements through the use of electrical signals/action potentials. This communication between our brain and the muscles in our body is what allows athletes like Damian Lillard to perform highly complex tasks, like shooting a step-back three pointer.
Other Resources
Here are some links to other resources that may help you better understand the way our brain controls our muscles!
Nervous Control of Muscle Contraction (https://www.youtube.com/watch?v=QItS-l2-pRg)
Bones, Muscles, and Joints (https://kidshealth.org/en/teens/bones-muscles-joints.html#:~:text=Muscles%20move%20body%20parts%20by,a%20limb%20at%20a%20joint)
References:
Ebashi, S., Endo, M., & Ohtsuki, I. (1969). Control of muscle contraction. Quarterly Reviews of Biophysics, 2(4), 351-384. doi:10.1017/S0033583500001190
Hodgkin, A. L., & Huxley, A. F. (1952). A quantitative description of membrane current and its application to conduction and excitation in nerve. The Journal of physiology, 117(4), 500–544. https://doi.org/10.1113/jphysiol.1952.sp004764
Hollyday, M., Hamburger, V., and Farris, J. M. (1977). Localization of motor neuron pools supplying identified muscles in normal and supernumerary legs of chick embryo. Proc. Natl. Acad. Sci. U.S.A. 74, 3582–3586. doi: 10.1073/pnas.74.8.3582
Mukund, K., & Subramaniam, S. (2020). Skeletal muscle: A review of molecular structure and function, in health and disease. Wiley interdisciplinary reviews. Systems biology and medicine, 12(1), e1462. https://doi.org/10.1002/wsbm.1462
